1. Field of the Invention
The present invention relates to a field sequential driving type liquid crystal display (FS-LCD) and, more particularly, to a method for driving a liquid crystal display with a digital driving type and an analog driving type mixed.
2. Description of the Related Art
In general, a color liquid crystal display includes an upper substrate, a lower substrate, a liquid crystal panel having the liquid crystal injected between the upper and lower substrates, a driving circuit for driving the liquid crystal panel, and a backlight for providing a white light to the liquid crystal. This liquid crystal display is classified into two types: a R, G and B color filter mode and a color field sequential driving mode based on the mode that color images are displayed.
The color filter type liquid crystal display divides one pixel into R, G and B subpixels. R, G and B color filters are arranged at the R, G and B subpixels, respectively. Light is delivered to the R, G and B color filters through the liquid crystal from one backlight to display the color images.
In contrast, the color field sequential driving type liquid crystal display has R, G and B backlights arranged at one pixel that is not divided into the R, G and B subpixels. Light of three primary colors from the R, G and B backlights is sequentially displayed at the pixel through the liquid crystal in a time-division manner, so that the color images are displayed by means of an after-image effect of the eyes.
The color field sequential driving type liquid crystal display sets a plurality of reference voltages corresponding to the number of gradations to be displayed, selects one reference voltage corresponding to the gradation display data among the plurality of reference voltages using an analog switch, drives the liquid crystal panel with the selected reference voltage, and performs the gradation display with an amount of transmission light corresponding to the applied voltage.
FIGS. 1A and 1B show waveforms explaining a method of driving the liquid crystal display performing a gradation display by varying the driving voltage of the liquid crystal in accordance with the prior art. In particular, FIGS. 1A and 1B show the waveforms with respect to the driving voltages applied to the liquid crystal and the amount of light transmitted to the liquid crystal based on the driving voltages.
Referring to FIGS. 1A and 1B, a driving voltage of V11 level is applied to the liquid crystal during a period T1 that ranges from the time t1 to the time t3, and light corresponding to the driving voltage of V11 level is transmitted to the liquid crystal. A driving voltage of V12 level higher than the V11 level is applied during a period T2 that ranges from t4 to t6, and an amount of transmission light corresponding to the driving voltage of V12 level is obtained. A driving voltage of V13 level higher than the V12 level is applied during a period T3 that ranges from t7 to t9, and an amount of transmission light corresponding to the driving voltage of V13 level is obtained.
The Red color is displayed during the period Tr that ranges from t2 to t3, which causes a Red light emitting diode of the R backlight to emit light, the Green color is displayed during the period Tg that ranges from t4 to t6, which causes a Green light emitting diode of the G backlight to emit light, and the Blue color is displayed during the period Tb that ranges from t8 to t9, which causes a Blue light emitting diode of the B backlight to emit light.
In the analog type driving method varying the above-mentioned driving voltages, there exist problems of tailing, blurring of colors, low contrast ratio, and stroboscopic motions. Furthermore, the analog type driving method displays the gradation with the degree of the driving voltage applied to the liquid crystal, which causes difficulty in implementing a fine gradation display.
To cope with the above-mentioned problems, Japanese Patent Publication Nos. 2003-98505, 2003-099015, and 2003-107425 disclose methods for displaying the gradation by means of digital control.
One method for displaying the gradation by means of digital control has a voltage applying time corresponding to the gradation to be written into a look-up table, reads out the voltage applying time corresponding to the gradation data from the look-up table, and applies a predetermined voltage to the liquid crystal during the voltage applying time corresponding to the gradation data to thereby perform the gradation display. This method makes the driving voltage applied to the liquid crystal constant, and controls the voltage applying time to perform the gradation display. As such, the driving voltage is kept constant and the voltage applying state and the voltage non-applying state are controlled with respect to their timings, so that a response time of the liquid crystal based on the gradation level can improve.
Another method for displaying gradation by means of digital control has an applying pattern corresponding to the gradation written into a look-up table, reads out the applying pattern corresponding to the gradation data from the look-up table, and applies a predetermined level of driving voltage to the liquid crystal based on the read applying pattern within a light emitting unit period of a light emitting diode to thereby perform the gradation display. This method makes the applying pattern be varied within the light emitting unit period of the light emitting diode and the voltage applying state and the voltage non-applying state controlled with respect to their timings. As such, the gradation display is performed based on the voltage applying time, so that a response time of the liquid crystal can improve.
Yet another method for displaying the gradation by means of digital control has an area corresponding to each gradation, wherein the area results from integrating the waveform of the amount of light transmitted to the liquid crystal with a light emitting period of the light emitting diode when the driving voltage is applied to the liquid crystal, and then varies the area to perform the gradation display.
The method for employing the integration of the amount of transmission light as mentioned above sets the voltage applying time in consideration of the area obtained from integrating the amount of transmission light with the light emitting period of the LED, so that a fine gradation display suitable for the gradation display is possible, and the waveform of the amount of light transmitting the liquid crystal is drastically increased or decreased improving the response time of the liquid crystal.
FIGS. 2A and 2B show waveforms explaining a method of driving a digital driving type liquid crystal display of the prior art. In particular, FIGS. 2A and 2B show the waveform of the driving voltage based on the driving data having a predetermined bit and the waveform of the amount of light transmitted to the liquid crystal based thereon.
Referring to FIGS. 2A and 2B, driving data corresponding to each gradation are supplied as a digital signal having predetermined bits, for example, seven bits, and a driving voltage based on the driving data having the seven bits is applied to the liquid crystal. The amount of light transmitted to the liquid crystal is determined based on the applied driving voltage to thereby perform the gradation display.
However, in the conventional digital type driving method as mentioned above, the number of bits of the driving data should be increased so as to implement a full color gradation display with a fast response time. In the meantime, in the liquid crystal display employing the field sequential driving method, since R, G and B light emitting diodes are sequentially driven in a time-division manner as compared to the conventional liquid crystal display, a higher driving frequency is employed as compared to the conventional liquid crystal display. As such, when the number of bits of the driving data is increased to implement the full color gradation display with the fast response time, the driving frequency increases more and more.
As such, when the driving frequency is increased, distortion from a gate driving voltage and a common power source voltage (Vcom) occurs, causing degradation in image quality. Furthermore, the liquid crystal is driven by the high driving frequency at a fast speed, causing an increase in power consumption. In addition, in the conventional digital driving method, the effective value response of the current gradation to be displayed is affected by the gradation just previously displayed, which causes difficulty in performing a fine gradation display. In particular, when the intermediate gradation is to be displayed, the gradation that has been just previously displayed significantly affects the current gradation to be displayed.
To cope with the above-mentioned problem that the effective value response is affected by the just previously displayed gradation in the digital type driving method, U.S. Pat. No. 6,567,063 discloses a method for digitally displaying the gradation using reset pulses.
FIGS. 3A through 3F show waveforms explaining a method of driving a digital type LCD using reset pulses in accordance with the prior art. Referring to FIG. 3, a plurality of periods T31 to T36 are employed to drive each of the R, G and B light emitting diodes for R. G. B backlights to thereby perform the gradation display.
A predetermined voltage VLC based on the R gradation data is applied to the liquid crystal in the T31 period, and the light transmitted by the liquid crystal is based on the applied voltage, so that the R light is displayed in the period where the R light emitting diode emits light. A predetermined voltage VLC based on the G gradation data is applied to the liquid crystal in the T32 period, and the light transmitted by the liquid crystal is based on the applied voltage, so that the G light is displayed in the period where the G light emitting diode emits light. In the mean time, a predetermined voltage VLC based on the B gradation data is applied to the liquid crystal in the T33 period, and the light transmitted by the liquid crystal is based on the applied voltage, so that the B light is displayed in the period where the B light emitting diode emits light. Thus, a color having a predetermined gradation is displayed.
In the above-mentioned digital driving method, a predetermined voltage is applied, which is different from the absolute value of the gradation data and has no relation with the gradation data during a predetermined time (i.e., each of t31 to t36) at the point where each of the periods T31 to T36 end. Thus, after R, G and B colors having a predetermined gradation are displayed at each of the period T31 to T36, a voltage that has no relation with the gradation data is supplied at the end point of each period so that no light is transmitted. Thus, when the liquid crystal is driven by the applied voltage based on the gradation data at each of the periods T31 to T36, the current period is not affected by transmission and liquid crystal state of the previous period so that the response time of the liquid crystal may be improved. In this case, the signal applied at the end point of each period T31 to T36 is referred to as a reset pulse, which improves the response time of the liquid crystal.
Thus, the digital gradation displaying method using the above-mentioned reset pulse has the advantage that the response time of the liquid crystal is improved to implement moving pictures. However, this method should allocate a predetermined portion of driving data bits to the reset pulses, which causes a significant increase in the bit number of the driving data as compared to the conventional digital driving method. When the bit number of the driving data increases, the above-mentioned driving frequency also increases increasing power consumption, and distortion from the gate voltage and the common voltage also causes degradation in the image quality.
Thus, when the liquid crystal display is driven in a digital manner, a gate pulse width with not less than a threshold value should be maintained, which limits fast driving and increases the frame frequency for preventing flicker. As such, a reverse driving method cannot be applied to improve the image quality, which causes cross talk, flicker, or the like.